Why is my cloth simulation jagged?
Try increasing the number of Steps under the cloth's Quality setting. Don't be afraid to increase it 3 or 4-fold: the simulation time will just increase proportionally (it will take 3 or 4 times longer to simulate), not exponentially (like with sub-surface steps).

When you increase the Steps count to - say - 15, each frame is split by the simulation engine in 15 sub-frames for calculation purposes. This is very useful when the collision/pinning objects have sudden accelerations, because it allows for the engine to deal with them in a finer way, in this case one fifteenth at a time.

Here's a comparison between a Quality = 2 steps and a Quality = 30 steps simulation. As you can see, the sudden accelerations create persistent and unrealistic wrinkles in the low-quality cape, while the high-quality one effectively and rapidly smooths them out (please note: my cape only has the 100% pinning on the top edge, and not the additional 10% one).



Additionally, you may want to increase the Quality value in the Cloth collision panel, but this isn't as relevant as the cloth quality.